1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a semiconductor device including a high-permittivity insulating film and a metal gate electrode. The present invention particularly relates to technology for enhancing performance of MOSFETs (Metal Oxide Semiconductor Field Transistor).
2. Description of the Related Art
Advanced CMOS (complementary MOS) device development with increasing miniaturization of transistors faces problems of a deterioration of drive current due to a depletion of a polysilicon (Poly-Si) electrode and an increase in gate current due to the thinning of a gate insulating film. To cope with these, studies are in progress on combined technology for achieving both the prevention of the depletion of the electrode by employing a metal gate and the reduction in gate leak current by forming the gate insulating film with a larger physical film thickness using a high-permittivity material. Pure metals, metal nitrides, silicide materials, and the like are studied as a material for the metal gate electrode. Whichever material is used, the threshold voltages (Vth) of n-type MOSFETs and p-type MOSFETs need to be settable at their proper values. When a transistor uses a conventional gate electrode of a polycrystalline silicon film, the threshold voltage of the transistor is determined by the impurity concentration in a channel region and the impurity concentration in the polycrystalline silicon film. On the other hand, when a transistor uses a metal gate electrode, the threshold voltage of the transistor is determined by the impurity concentration in a channel region and the work function of the gate electrode. To manufacture CMOS transistors with a Vth of ±0.5 V or smaller, the gate electrode for n-type MOSFETs needs to use a material having a work function of the mid-gap of Si (4.6 eV) or lower, desirably 4.4 eV or lower, and the gate electrode for p-type MOSFETs needs to use a material having a work function of the mid-gap of Si (4.6 eV) or higher, desirably 4.8 eV or higher. A CVD (Chemical Vapor Deposition) method, a PVD (Physical Vapor Deposition) method, or the like is used as means for forming such metal gate electrodes.
A description will be provided here of an example of a method of forming a metal gate electrode using the CVD method. Non-patent Document 1 uses the CVD method to form a metal gate electrode and succeeds in achieving a high effective work function (5.1 eV) and good leak current (Jg) characteristics relative to an equivalent oxide thickness (EOT). However, it is conceivable that transistor characteristics deteriorate due to impurities contained in a material of the electrode, as shown in Non-patent Document 2. On the other hand, the PVD method possibly causes a smaller amount of impurities to be mixed in the material than the CVD method does, but has problems that the leak current characteristics deteriorate and a desired effective work function cannot be obtained unlike the CVD method, as shown in Non-patent Document 3. To address these problems, Patent Document 1 reports that leak current characteristics are improved by forming an electrode having a laminate structure of metal nitride and metal with nitriding of a surface portion of an insulating film (oxide) by a DC sputtering method. However, this method is becoming inadequate with advancement of the miniaturization of transistors (a gate length of 32 nm or smaller).